4500 auto gu

Not sure of what you wish to achieve overall Philip - but some of these things can be counter productive so I'd have a well thought out target .

Exhaust usually makes petrol car worse for 4wding where low down torque is the aim , as opposed to power at 5000 rpm which is great for drag racing.

My current daily driver is a 4800 auto - when I want it to go faster I put on my smaller wheels and keep tyre pressures up.

Over the years I'm pretty sure I have tried / used/ made every mod available.

Currently we still have a heavily blown 4800 delivering 400kw which smokes the kids new 300kw Y62.

So my overall advice is to think longer term and that small mods are basically a waste of time.

Plan to upgrade to a stock standard 4800 - and you get an immediate 30% real gain across the board with better fuel consumption which is factory supported and more reliable for those out of the way trips.

Philip
If you can feel the aircon load on a Patrol engine there must be something wrong. They only take a small amount of power, even small cars with very small engines have aircon and they do OK, but you can feel it in them.

Perhaps you have an intake airlfow problem or the tuning isn't good. Headers and exhaust aren't going to do anything for normal use power. Bigger pipes for the gas to run down means it travels slower and provides less extracing effect at normal speeds, ok up higher as Robin mentioned.
Is the engine timed correctly for the fuel octane it is burning. 98 octane burns slower than 91 unleaded and therefore the timing should be set for the best performance. 98 requires more initial advance, something which modern engines do automatically with knock sensor systems. A decent air filter may be an advantage.

If you want a surge of power you can always hit the aircon OFF button. With the increase in power it will be cool in another way.